As a satellite position measurement technology for receiving and analyzing radio waves from a plurality of satellites and detecting a position of a receiver (hereafter called “mobile station”), a single position measurement system for measuring a position by a solo mobile station even if the measurement error is large, and a relative position measurement system for accurately determining the position of a mobile station using the correction data from a reference station of which position is known, are available.
In the relative position measurement system, the position measurement accuracy is restricted by the linear distance between receivers, which is the so called “base line length”.
For example, in the case of a relative position measurement system which measures the position independently using the C/A code of GPS (Global Positioning System) and correcting the position using correction data, the limit of the base line length, when the uncertainty of the orbit information of the satellite and the error due to the ionosphere and atmosphere or the like are approximately the same for each receiver, is about 100 km or less, and an error can be canceled within this range, so an improvement of the position measurement accuracy in the relative position measurement system can be expected.
On the other hand, in the case of a system which analyzes the base line using a carrier phase so as to improve the position measurement accuracy, the limit of the base line is short, about 10 km or less, but the position measurement accuracy improves dramatically since a carrier phase, which is sufficiently shorter than the C/A code, is used [e.g. see the new edition of GPS—Precision Positioning System by Satellites, (issued by Japan Survey Association)].
Any of the above-mentioned relative position measurement systems, however, can be used in the range of the base line limit length from the reference station and in the area where the reference station is not organized, the relative position measurement cannot be used no matter where the mobile station is located, whether on land, ocean or in the sky, so position cannot be measured accurately.